(1) Field of the Invention
The present invention relates to an ink-jet printing system which includes a sheet transport mechanism in which a copy sheet at a print position is transported while an image is formed on the copy sheet by a print head.
(2) Description of the Related Art
In image forming systems, such as printers, facsimiles, or copiers, an image is created on a copy sheet. An ink-jet printing system is one of such image forming systems. In the ink-jet printing system, a print head forms an image on a copy sheet at a print position by spraying ink onto the copy sheet through a nozzle of the print head. The copy sheet may be a sheet of paper or a sheet of plastic material. In the ink-jet printing system, a sheet transport mechanism is provided to transport a copy sheet at a print position while the image is formed on the copy sheet by the print head. During the image forming, the print head moves across the copy sheet in a main scanning direction to draw one scan line, returns while the copy sheet is transported in a sub-scanning direction by one inter-scan-line spacing, and draws the next scan line.
Generally, in the above-described ink-jet printing system, the ink is sprayed onto the copy sheet, and slight irregularities in the flatness of the copy sheet that is transported during the image forming can arise. If the copy sheet is in a wavy or distorted condition during the image forming, the resulting image will contain a distortion. If the copy sheet is excessively slanted, the print head will be brought into contact with the copy sheet, which is detrimental to both the print head and the image quality. In order to obtain good quality of the reproduced image, it is desirable to stably hold the copy sheet in a flat condition relative to the print head during the sheet transport.
As disclosed in Japanese Laid-Open Patent Applications No. 7-125,364, No. 7-132,659 and No. 9-24,650, a sheet transport mechanism which uses rollers to pinch the copy sheet at upstream and downstream positions along a sheet transport path is known. However, when such rollers are arranged in an image forming system, an effective print range of the print head for the copy sheet will be too narrow due to the restriction of the layout of the entire system and the use of the rollers. As the rollers must be arranged adjacent to the print head, the top and bottom margins of the copy sheet where the image forming is impossible are relatively large.
Japanese Laid-Open Patent Application No. 7-125,364 discloses a dot-impact printer including first rollers, second rollers, and a flat-surface platen. A copy sheet is transported by the first and second rollers. The platen is arranged under the print head such that the platen and the print head confront each other via the copy sheet to print an image on the copy sheet against the flat surface of the platen. However, when such rollers and platens are arranged in the printer, an effective print range of the print head for the copy sheet will be too narrow due to the restriction of the layout of the entire system and the use of the rollers.
Japanese Laid-Open Patent Application No. 7-132,659 discloses a sheet ejection mechanism of an ink-jet printer. The copy sheet after the image is formed thereon is ejected out of the printer by an ejection roller and a spur roller. In order to avoid the ink stain in the copy sheet by the rollers which interfere with the copy sheet, the spur roller is arranged at a slightly upstream position from a position just above the outer periphery of the ejection roller. However, the mechanism of the above publication is inadequate to hold the copy sheet at the print position in a flat condition relative to the print head during the sheet transport.
Japanese Laid-Open Patent Application No. 9-24,650 discloses an ink-jet printer in which a copy sheet is transported along the periphery of a platen roller, and a part of the periphery of the platen roller is used as a print position where the ink is sprayed onto the copy sheet by the print head. A pinch roller and a star wheel are arranged at upstream and downstream positions on the periphery of the platen roller so as to press the copy sheet against the platen roller. The mechanism of the above publication is inadequate to hold the copy sheet in a flat condition relative to the print head during the sheet transport.
Further, a sheet transport mechanism which uses a transport belt to transport the copy sheet is known. For example, Japanese Laid-Open Patent Application No. 6-80,268 discloses a belt conveyer device which uses a transport belt running on rollers to transport a copy sheet within an image forming system. One of the rollers is improved to contain an elastic-material layer on a core shaft in order to reduce the deformation of the belt. The belt conveyer device of the above publication is applicable to various image forming systems such as printers, facsimiles or copiers. However, the belt conveyer device of the above publication is inadequate to hold the copy sheet in a flat condition relative to the print head during the sheet transport.
Japanese Laid-Open Patent Application No. 7-137,877 discloses a sheet transport mechanism in which a comb-form electrode pattern is provided under the surface of a transport belt. A bias voltage is supplied to the transport belt via the electrode pattern, so as to create an electrostatic attraction for the copy sheet which is transported by the transport belt. However, it is necessary that the transport belt comes in contact with the electrode pattern, and the load to drive the transport belt will be increased by the contact of the belt with the electrode pattern. The configuration of this mechanism is complicated and the cost will be raised. The conventional mechanism of the above publication is inadequate to avoid the occurrence of slight irregularities in the flatness of the copy sheet that is transported during the image forming.
Japanese Laid-Open Patent Application No. 4-31,070 discloses a sheet transport mechanism of an ink-jet printer in which a copy sheet is electrostatically attracted by a transport belt while it is transported. In order to reduce the load to drive the transport belt and maintain the speed of the belt at a constant level, a cleaning blade which is attached to the belt for cleaning purpose is detached from the transport belt when a start of image formation on the copy sheet is detected. However, the conventional mechanism of the above publication requires a sensor to detect the timing of the start of image formation and a control unit to detach the cleaning blade from the transport belt at the detected timing. The configuration of the sheet transport mechanism is complicated and such a mechanism is expensive.
An object of the present invention is to provide an ink-jet printing system including an improved sheet transport mechanism in which the above-described problems are eliminated.
Another object of the present invention is to provide an ink-jet printing system including a sheet transport mechanism which is constructed in a simple structure and stably maintains the copy sheet in a flat condition relative to the print head while transporting the copy sheet at the print position during the image formation.
The above-mentioned objects of the present invention are achieved by an ink-jet printing system including: a print head which forms an image on a copy sheet at a print position by spraying ink onto the copy sheet through a nozzle of the head; and a sheet transport mechanism which transports the copy sheet along a sheet transport path while the image is formed on the copy sheet by the print head, the sheet transport mechanism including a sheet guide member provided under the print head to guide the transport of the copy sheet, and including a sheet attracting member which pneumatically attracts the copy sheet to the sheet guide member during the image forming.
The above-mentioned objects of the present invention are achieved by an ink-jet printing system including: a print head which forms an image on a copy sheet at a print position by spraying ink onto the copy sheet through a nozzle of the head; and a sheet transport mechanism which transports the copy sheet along a sheet transport path while the image is formed on the copy sheet by the print head, the sheet transport mechanism including a transport belt which transports the copy sheet, the transport belt having a number of air vents formed therein, and the sheet transport mechanism including a sheet attracting member which pneumatically attracts the copy sheet to the transport belt, during the transport of the copy sheet, by generating a flow of air passing through the air vents of the transport belt.
In the ink-jet printing system of the present invention, the sheet transport mechanism is constructed in a simple structure, and it is effective in maintaining the copy sheet in a flat condition with respect to the print head while transporting it at the print position during the image formation. The limitation of an effective print range of the print head for the copy sheet due to the restriction of the layout of the entire printing system can be eliminated. It is possible to reliably prevent the copy sheet from being set in a wavy or distorted condition during the image forming, and this makes it possible to obtain good quality of the reproduced image.